A conventional active type optical distance measuring sensor is known as a device configured to irradiate light from a light source for projection of light such as an LED (Light Emitting Diode) toward an object, to detect reflected light from the object with a photodetecting element, and thereby to output a signal according to the distance to the object. For example, a PSD (Position Sensitive Detector) is known as an optical distance measuring sensor of an optical triangulation type capable of readily measuring the distance to the object, and in recent years there are expectations for development of an optical distance measuring sensor of an optical TOF (Time-Of-Flight) type, in order to achieve more accurate distance measurement.
There are demands for an image sensor capable of simultaneously acquiring distance information and image information by a single chip, for example, in on-vehicle use, use in automatic manufacture systems in factories, and so on. With the image sensor being installed in the front portion of a vehicle, its expected use is to detect and recognize a preceding vehicle or to detect and recognize a pedestrian or the like. There are also expectations for an image sensor capable of acquiring a range image consisting of a single piece of distance information or multiple pieces of distance information, separately from the image information. It is preferable to apply the TOF method to such a distance measuring sensor.
In the TOF method, pulsed light is emitted from a light source for projection of light toward an object and the pulsed light reflected by the object is detected by a photodetecting element to measure a time difference between the emission timing and the detection timing of pulsed light. This time difference (Δt) is a time for the pulsed light to travel a distance (2×d) twice as long as the distance d to the object, at the speed of light (=c), and therefore the relation of d=(c×Δt)/2 holds. The time difference (Δt) can be translated into a phase difference between pulses emitted from the light source and detected pulses. The distance d to the object can be obtained by detecting the phase difference.
An image sensor of a charge distribution type has been attracting attention as a photodetection element for distance measurement by the TOF method. Specifically, the image sensor of the charge distribution type is configured, for example, to distribute a charge generated in the image sensor according to incidence of a detected pulse into one potential well during an ON duration of an emitted pulse and into the other potential well during an OFF duration. In this case, a ratio of charge quantities distributed right and left is proportional to the phase difference between the detected pulse and the emitted pulse, i.e., the time necessary for the pulsed light to travel the distance twice as long as the distance to the object at the speed of light. There are various conceivable methods of the charge distribution type.
Patent Literature 1 discloses the range sensor (range image sensor) of the charge distribution method, which comprises a semiconductor substrate, a photogate electrode provided on a surface of the semiconductor substrate and having a planar shape with two sides opposed to each other, a plurality of gate electrodes provided respectively next to two sides of the photogate electrode on the surface, and a plurality of semiconductor regions having the conductivity type different from that of the semiconductor substrate and provided for reading out respective charges flowing from a region immediately below the photogate electrode into regions immediately below the respective gate electrodes.